The present invention relates to acoustical devices and particularly to an ultrasonic sound absorbing material or baffle. The sound absorbing baffle is particularly useful in acoustical logging instruments used for measuring characteristics of formations that are penetrated by boreholes drilled in the earth. Acoustical logs that represent acoustical properties of formations are extensively used in evaluating formations. In addition to general acoustical properties of formations, acoustical logging methods have also been used to detect vertical fractures in formations. Fractured formations are difficult to locate and yet they provide a valuable source of hydrocarbons.
An acoustical tool that is designed to locate vertical fractures is described in U.S. Pat. No. 4,130,816. This patent describes the use of four cylindrical transducers disposed in a horizontal plane and positioned in close proximity to the borehole wall. Two of the transducers are used as transmitters while two are used as receivers to measure the circumferential acoustic characteristics of the formations. The tool is designed to detect both the compressional and the shear waves that travel horizontally or circumferentially around the borehole wall. Shear waves lose much of their amplitude when they traverse an open space, such as a fracture, and by observing those locations where the amplitude of the shear wave is materially decreased, one can locate or detect the presence of fractured formations.
One problem with the above described logging tool has been the interference caused by waves which travel directly from the transmitter through the borehole fluid to the receiver. These waves are often of larger amplitude than the compressional and shear waves which have travelled circumferentially around the borehole through the formation. These waves are often referred to as fluid waves and they arrive at the receiver at approximately the same time that the shear waves which travel through the formation. In an attempt to limit or decrease the effect of fluid waves on the receiving transducer, various acoustical absorbing materials have been placed around the cylindrical transducers. The absorbers are designed to block sound energy that tends to travel through the fluid and arrive at the receiver from a direction opposite the borehole wall. These absorbers include various types of moldable plastic materials which are filled with lead or similar heavy metals. While the absorbers have improved the response of the tool they have not entirely eliminated the interference from fluid waves since they do not completely absorb the fluid waves.